Jon's mother, while oblivious or obstinate on most topics, responds with such exaggeration to news of Jon's steady dating that the reader is nudged to remember exactly how monumental a development it is. By keeping Mom off-stage, we're allowed to read the entire spectrum of possible responses, from "I am happy; finally my son will begin a family" to "the bottom has dropped out of my reality". Because all that we know of Mrs. Arbuckle's reaction is that her circuit breakers have been tripped - she could be shocked, delighted, etc. - perhaps the reactions we personally migrate towards can tell us something about our own responses to Jon, or even about our relationships with our own parents and how we feel they view us.
Another dimension to Jon's mother's surprise is underlined by the secondary joke that the Arbuckle family is so cornpone that there is always a big helping of mashed potatoes in the immediate vicinity. The generational and cultural gap between Jon and his family likely means Jon's prolonged bachelorhood has seemed even more extreme to his rural parents.
Garfield of course has zero interest in either Jon's love life, or Arbuckle family business, and latches onto one tangential idea for his own personal punchline, "I've had dreams like that." It's a double-barrel joke, telling us A) that the Arbuckles are so boring/ Garfield so self-absorbed that he's not even paying attention, B) Garfield's gluttony extends into a deeply confused place in his subconscious. It's one thing to have a wish-fulfillment dream about diving into a swimming pool of mashed potatoes, or eating your way out of a cave made of mashed potatoes... but fainting unconscious into food means that in the dream you're not even eating, but being covered, smothered, consumed. This wiggles past defiant indulgence, or even food addiction and into fetishism that will take expert psychologists with more serious training than I to untangle.
Optional Reader Activity Worksheet: Call your mother and inform her that Jon Arbuckle has a girlfriend. What is her response? Into what food does she pass out? Will real cats even eat potatoes?